Process for manufacturing ammonium salts and magnetic iron oxide from solutions of ferrous salts



Nov. 14, 1950 R. D. HOAK PROCESS FOR MANUFACTURING AMMONIUM SALTS AND MAGNETIC IRON OXIDE FROM SOLUTIONS OF FERROUS SALTS Filed Sept. 27, 1947 AIR - nnnlnuunuunnnnnnnunuununm Jag .1.

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- REACTOR PICKLE AMMONIA LIQUOR SUPPLY SUPPLY nnnuunnunnunnuununnum nllnnnulllllllilnllnnllllnllnnnl INVENTOR RICHARD D. HOAK dioxide.

Patented Nov. 14, 1950 PROCESS FOR MANUFACTURING AMMO- NIUM SALTS AND MAGNETIC IRON OXIDE FROM SOLUTIONS OF FERROUS SALTS Richard D. Hoak, Mount Lebanon Township, Allegheny County, Pa'., assignor to Mellon Institute of Industrial Research, Pittsburgh, Pa., a corporation of Pennsylvania Application September 27, 1947, Serial No. 776,504

Claims. 1

This invention relates to a process for producing pure ammonium salt and a rapidly settling magnetic iron oxide from waste pickle liquor, or liquor of similar composition, and purified ammonia as or aqua ammonia.

Spent pickle liquor, a waste product of the steel industry, is produced in very large volume from an operation whereby the oxide scale is removed from steel products, prior to further processing, by immersing them in a bath of dilute sulphuric acid. This liquor is predominantly an aqueous solution of ferrous sulphate and free sulphuric acid. The composition of waste pickle liquor will normally fall in the range of to FeSO; and 0.5% to 10% H2804. of compositions similar to that of waste pickle liquor is produced in large volume in the manufacture of certain pigments, especially titanium Some steel products are pickled with hydrochloric acid, for example, steel wire, and the resultant spent pickle liquor contains ferrous chloride and hydrochloric acid.

The economical disposal of waste ferrous sulphate solutions, as for example, waste pickle liquor, or liquors of similar composition, constitutes a serious problem. In many cases no alternative remains butto treat the liquor with lime, or some other alkaline agent, to prevent its discharge into streams where it would become an objectionable pollutant. This is a costly procedure because no by-product of value is obtained. A process whereby waste pickle liquor can be profitably employed is obviously of public benefit;

Large quantities of ammonia are produced in the coking of coal. This chemical is converted into ammonium sulphate for use in the chemical industry, primarily as a component of fertilizers, by absorption in sulphuric acid. The provision of a more economical source of sulphate ion would be of great advantage to the industry. This advantage would be enhanced if the resulting ammonium sulphate were of higher quality than can be produced by conventional means.

This invention relates to a process for treating coke oven ammonia gas or aqua ammonia, purified of its contaminants (notably sulphides and cyanides) by known means, with waste pickle liquor or liquor of similar composition, whereby the sulphate component of the waste pickle liq uor is substituted for the sulphuric acid conventionally employed to manufacture ammonium sulphate from gaseous ammonia.

I am aware of the prior art, wherein waste pickle liquor has been treated with ammonia to A liquor recover hydrated iron oxides and ammonium sulphate by diverse means. These prior processes, however, operate either under superatmospheric conditions or on the batch principle, or both. The present invention relates to a novel method for completely precipitating the iron in waste pickle liquor at a high rate, by a continuous process or by a cyclic continuous process, to form a magnetic iron oxide of superior settling quality, by reaction with ammonia and air, the process being carried out at atmospheric pressure.

One of the obstacles to the successful operation of the prior processes has been the difliculty of producing the hydrated iron oxides in an easily filterable form. This also made it impossible economically to recover the ammonium sulphate solution mechanically held in the oxide sludge. This impediment is overcome in the present invention by producing the oxides in such physical form that very rapid settling occurs and provides for recovery of the entrapped salt solution by the well-known method of continuous decantation. This is an important advance in the art.

Where a solution of a ferrous salt is treated with an alkaline agent, complete precipitation of the ferrous iron cannot ordinarily be effected until the pH is raised above 9. In accordance with my invention, an aqueous slurry of ferrous sulphate and ammonia is agitated and one or more streams of air are fed to the slurry to oxidize ferrous iron to ferric iron. A particular merit of the invention is that it includes thediscovery that, by proper proportioning of the reactants and by feeding air thereto in proper amounts. arapid settling magne ic iron oxide, largely hydrated ferroso-ferric oxide is formed, which accomplishes complete precipitation .of the iron in the liquor at a pH of 7.5 to 8.0. This will be recognized as an important discovery since it provides for the complete precipitation of ferrous iron at a pH value. lower than that at which ferrous iron can normally be precipitated completely.

The hydrated iron oxide recovered by my process can advantageously be sintered for charging to blast furnaces or can be used as a pigment or by suitable treatment can be converted into other iron oxide pigments.

In the accompanying drawings which illustrate, in a somewhat diagrammatical manner, one form of apparatus for carrying out the process,

Figure 1 is a diagrammatic illustration of the system as a whole;

Figure 2 is a plan view of a shrouded impeller;

Figure 3 is a vertical section taken on the line III-III of. Figure 2; and

Figure 1 illustrates the shearing action of the shrouded impeller blades.

My process is based upon a controlled oxidation of the ferrous hydroxide resulting from the reaction between ammonia and an aqueous solution of ferrous sulphate. streams of waste pickle liquor and gaseous or aqua ammonia are fed to a reaction chamber equipped with an agitator ofsuch design as will provide for eflicient dispersion of a stream, or streams, of air admitted thereto. The operation of the process may be either continuous or cyclic continuous. The latter method of operation permits a higher conversion of waste pickle liquor per unit of time than the former, as will be shown hereinafter.

The reaction is carried out at a temperature between about 50 and about 100 C., the preferred temperature being about 65 C. and the preferred range being between 55 and 80 C. If the temperature is too low the process will not operate satisfactorily. On the other hand, there is no harm in operating at temperatures above the preferred temperature of 65 C. except that as the temperature increases the tendency to boil off ammonia increases. The temperature of the reactants may be raised initially as by injection of steam. During the normal operation the reaction is sufliciently exothermic to maintain the preferred temperature of 65 C. and it may even be necessary to provide cooling means for maintaining the preferred temperature of reaction.

The process may be conducted in a cylindrical vessel which is relatively tall in proportion to its diameter. A tank whose height is approximately three times its diameter is preferred but the relation of height to diameter is not limited to this proportion.

Referring more particularly to the accompamring drawings, a cylindrical reaction vessel 2 is equipped with suitable means for providing agitation and aeration. As shown, a vertically extending impeller shaft 3 is driven by a motor 4. Shrouded impellers 5 are fixed to the shaft 3. As shown in Figures 2 and 3, each impeller has a hub 6 which fits on the shaft 3. Curved impeller blades 1 are secured to this hub. Two annular rings 8 and 9 are secured to the top and bottom of the blades, each ring providing an opening l adjacent the outside of the hub. The impeller also is open at its periphery H. Ammonia gas or aqua ammonia is supplied through a pipe I2;

flows through a flowmeter l3 and a valve I4, and is delivered adjacent the bottom of the reaction vessel 2. Spent pickle liquor is supplied through a pipe l5, flows through a flowmeter l6 and valve I61; and is delivered to the reactor adjacent its bottom. Compressed air flows through a pipe l1,

flowmeter l8 and pipe Is. It then flows through branch pipes 20 controlled by valves 2| and is delivered to the reactor 2 adjacent the opening ID of each of the shrouded impellers which thoroughly mix the air and slurry and discharge the mixture from the periphery of the impeller. The reactor is provided with an overflow pipe 22 adjacent its top, a pipe 23 and valve 24 for use in carrying out the cyclic continuous process hereinafter described and a drain pipe 25 provided with valve 26. The reactor also is pro- Suitably proportioned vided with a vertical bailie 21 fastened to the wall and extending the full depth of the reactor to provide turbulence.

The action of the shrouded impeller 5 is illustrated in Figure 4. Asthe impeller rotates in theliquid and air is supplied to it from the pipes 20,- a layer of liquid 30 containing bubbles of air 3| is forced outwardly along the impeller blade I.

As each succeeding portion of the layer of liquid which contains a bubble of air reaches the outer edge 32 of the blade, the edge of the blade shears the layer and converts the bubble into a thin film of air 33 thereby increasing the rate of diffusion of the oxygen of the air into the liquid. This rate of diffusion is much greater than can be obtained by diffusing air into a liquid by means of a diffuser element composed of a porous material such as porous carbon. Furthermore, porous difiuser elements clog up in use.

The process can be carried out by a continuous method or by a cyclic continuous method.

The continuous method is as follows: Waste pickle liquor and ammonia gas pass through the flowmeters l5 and I3 which proportion their rate of flow in such a manner that there is always present a slight excess of ammonia, for example, 2.5% excess ammonia above the stoichiometric requirement of the waste pickle liquor. Aqua ammonia may be used in place of ammonia gas, if desired. These streams of reactants are fed to the bottom of the reactor, the shaft 3 is driven at such a rate that the peripheral velocity of the impeller blades I is approximately 700 feet per minute and air is fed to the impellers through the air supply pipes 20. As the reactor fills, live steam is admitted as occasion demands to raise the temperature of the reaction mixture above 50 C. and preferably to a temperature of about 65 C. As the reactor fills with slurry, the pickle liquor, ammonia and air feeds are so regulated that the atomic ratio of ferric iron to ferrous iron is within the range of 2:1 to 3.5: 1, the preferred ratio being between 2:1 and 25:1. When the reactor is filled, the slurry overflows from pipe 22. l

The object in controlling the ratio of ferric iron to ferrous iron is to produce a rapidly settling magnetic iron oxide which consists largely of hydrated ferroso-ferric oxide having the formula lFezOalFeOJiHzO. It will be noted that in ferroso-ferric oxide the atomic ratio of ferric iron to ferrous iron is 2:1. The atomic ratio of ferric iron to ferrous ir'on must be at least 2:1 in order to insure the precipitation of all .iron from the solution. However, as the ratio of ferric iron to ferrous iron increases substantially above 2:1, that is, to a value greater than 3.5:1, the precipitated magnetic iron oxide tends to settle more slowly. Within the atomic ratio of ferric iron to ferrous iron of 2:1 to 35:1, all of the iron is precipitated, thereby producing an ironfree ammonium sulphate solution and the magnetic iron oxide settles rapidly. In carrying out the process it is desirable to maintain the atomic ratio of ferric iron to ferrous iron only slightly above 2:1, say between 2:1 and 25:1, or better still between 2:1 and 22:1. This results in a the combined strong liquor and wash water by evaporation.

The process may be carried out in a cyclic continuous manner instead of by the continuous operation which has just been described. It has been found that the average rate of conversion of pickle liquor can be increased substantially by operating in a cyclic continuous manner as follows: The reactor is drained by opening the valve 24 until it is approximately half full of slurry from a previous run. Pickle liquor, ammonia and air feeds are turned on and the feeding is continued until the level of slurry has risen almost to the overflow pipe 22. Ammonia and pickle liquor feeds are then turned off but the oxidation of the slurry is continued by maintaining the air feed until the atomic ratio of ferric iron to ferrous iron in the hydrated oxide is slightly over 2:1. This ratio may be anywhere between 2:1 and 3.5:1. A portion of the slurry is then removed from the reactor by drainirig it to the half-way mark. the portion of the slurry removed from the reactor is allowed to settle, the supernatant decanted, the sludge washed and ammonium sulphate is recovered by evaporation of the combined strong liquor and wash water as previously described. The portion of slurry remaining in the reaction vessel is then utilized for repeating the cycle which has been described.

While in the cyclic continuous process it is necessary to start with a quantity of slurry from a previous run, it is not necessary that the quantity from the previous run constitute approximately one-half of the total slurry treated in this process. Furthermore, in the preferred method as described, the feed of ammonia and pickle liquor was stopped completely during the oxidation period in which the atomic ratio of ferric iron to ferrous iron was raised to over 2:1. During this oxidation period it is not necessary to stop the feed of ammonia and pickle liquor completely. They could be continued during this period provided the rate of air supply is sumcient to raise the atomic ratio of ferric oxide to ferrous oxide above 2:1. However, if the pickle liquor and ammonia feeds are not shut ofl' completely, the process will be less eiilcient.

In carrying out my invention by the cyclic continuous process I employ a feed period followed by an oxidation period. During the feed period I form a bath by feeding ammonia and an aqueous solution of ferrous sulphate to areaction zone while supplying air to the bath to oxidize some but not all of the iron to ferric iron. The rate of feed of ferrous sulphate solution during the feed period is such that a substantial amount of iron remains in solution in the bath at the end of the feed period. The amount of iron in solution at the end of the feed period is preferably between .5 gram and lgram per liter of solution, but it may be as low as .1 gram per liter or as high as 5 grams per liter. Following the feed period, the bath is subjected to an oxidation treatment, during which .the bath is agitated and air is supplied to it. but the feeds of ferrous sulphate solution and ammonia are cut off. The oxidation treatment is continued until the atomic ratio of ferric iron to ferrous iron in the precipitated oxide is between 2:1 and 35:1 and until substantially no iron remains in solution, by which I mean that at the end of the oxidation period the solution contains not over .05 gram of iron per liter.

4 in run No. 1 the oxidation period is 20.3% of the.

u with an oxidation period which was 63.5% of the The oxidation period may vary over a considerable range depending upon the amount of iron left in solution at the end of the feed period and depending also upon the quality of the oxide to be produced. Assuming a fixed air feed rate, where pickle liquor is fed to the reactor at a relatively low rate, the supernatant from the oxide product will contain no unprecipitated iron at the end of the feed period. As the pickle liquor feed rate is increased, a point is reached at which unprecipitated iron is present in the supernatant at the end of the feed period, necessitating further oxidation to effect its precipitation. This is accomplished by stopping the pickle liquor and ammonia feeds while the air feed is continued until substantially all of the soluble iron has been precipitated by oxidation to ferric hydrate. As the pickle liquor feed rate is increased further, the soluble iron in the supernatant at the end of the feed period will increase in proportion, requiring a corresponding increase in the length of the oxidation period. As the pickle liquor feed rate continues to increase, the pickle liquor conversion rate also increases but at a lower and lower rate because the oxidation period becomes a greater andgreater proportion of the total cycle time. Eventually, for a given air feed rate, a point is reached at which the pickle liquor conversion rate becomes a maximum.

As the point of maximum pickle liquor conversion rate is approached, however, the quality of the oxide product, as measured by its settling rate, begins to deteriorate. This is illustrated in the following tabulation:

Referring to runs No. 1 and No. 2, it is seen that feed period, whereas in run No. 2 it is 25.4%. It is to be noted further that the quality of the oxide product, as shown by the settling rate, was substantially better in the case of run No. 1 than for run No. 2. On the basis of extensive experimental work, I prefer to hold the oxidation period to a value not over 25% of the length of the feed period, but I do not limit my method of operation to such a value. The process has been operated feed period but the quality of the oxide was not as good. On the other hand, an oxidation period as low as 5% of the feed period resulted in an oxide product of high quality and a markedly increased waste pickle liquor conversion rate as compared with a process in which the feed rate was so low that'no subsequent oxidation treatment was required. In carrying out my invention the oxidation period is always of such length of timethat substantially no iron remains in solution at the completion of this period. If, in my process, the oxidation period is relatively short. the conversion rate for the overall process is relatively small, but the quality of the oxide produced is relatively high. On the other hand, if, in my process, the oxidation period is relatively long, the conversion rate for the overall process is relatively high but the oxide product tends to be of poorer quality, as shown by its decreased settling rate. While in carrying out my cyclic continuous process the oxidation period may be as low as or as high as 63% of the feed period, I prefer that it be between and 30% of the feed period.

The data from a typical cycle for the cyclic continuous method of operation as obtained from a reactor with a capacity of 25 liters are given in Table I below. The reactant rates for this cycle were: waste pickle liquor (containing 60 grams iron per liter). 320 ml./min.; air, 17.1 l./min.; ammonia, 33.55 l./min.; the gases were measured at 70 F. and 760 mm. of mercury.

Table I.--C'yclic continuous operation The figures given in the table refer to measmaximum conversion rate was 180 ml./min. This is equivalent to a 30 per cent increase in capacit" Wilson Patent No. 2,419,240 discloses treating waste pickle liquor with ammonia solution to produce a precipitate of ferroso-ferric hydrate. This patent emphasizes that in carrying out the reaction, the dilution of the ammonium salt must be maintained so high and the iron salt must be introduced into the bath so slowly that substantially no iron exists in soluble form in the bath. My invention provides a method whereby waste pickle liquor can be converted at a much higher rate than by the process of the Wilson patent, which, of course, is a distinct advantage. While the Wilson patent states that the process of his Example I results in a clear water-white solution of ammonium sulphate which showed no test for soluble iron when it was made ammoniacal and treated with hydrogen peroxide, I have duplicated his Example 1, carrying out the reaction in a 4 liter beaker, and have found that the resultant ammonium sulphate solution contained about .06 gram of iron per liter. I

I have also carried out tests in accordance with Wilsons Example I, employing the reactor used in the development of my process. For carrying out the example of Wilsons process in this reactor, the agitator of Figure 1 was removed and two diifuser stones were installed for dispersing the air fed to the reactor. Pickle liquor and ammonia were fed to the bottom of the reactor as indicated in Figure 1.

Wilson fed pickle liquor to a 4000 volume reactor at a rate of volumes per'minute. On the same basis, my reactor had a capacity of 25,000 volumes and the rate of pickle liquor fed to it should then have been 125 volumes per minute (25000/4000X20=125).

Two tests of Wilsons example were made. In the first test the pickle liquor feed rate was 100 ml./min., and in the second, 75 ml./min. The data referring to the tests are as follows:

Acid value, pickle liquor, 200.4 g. 804/1. Iron value, pickle liquor, 60.0 g. Fe/l. Ammonia solution, 155.9 g. NHs/l.

8 Volume 01' water in reactor at start of test,.6.5 1. Volume of ammonia solution in reactor at start of test, 1.5 l.

Test No. 1 Test N0. 2

100 ml./min.

50 37.5 ml./min. 25 25 l./min. 75 93 min. 54 57%. Temperature 75-80 75B0 C Settling rate of oxide (per cent Settled 91 90.

in 5 min. Flerric/Ferrous ratio of oxide 2813 3386-301 Eon in ammonium sulphate solution 0. 82 0.00 g./1

These data show that at a pickle liquor feed rate of mL/min. it was not possible to produce an iron-free supernatant. To do so it was necessary to reduce this rate to 75 ml./min.

The data given by Wilson for his Example I show that he fed ammonia at a rate more than 50% higher than the stoichiometric requirement of the pickle liquor feed. But in his specification, Wilson states tha the mixture need not have a pH much greater than 7, and satisfactory results are obtained when only the faintest odor of ammonia from the reaction mixture is detectable. I have, in consequence, duplicated his Example I, with the exception that the amount of ammonia fed was slightly more than 2% in excess of the stoichiometric requirement of the pickle liquor fed. Two tests were made, test No. 3 with a pickle liquor feed rate of 75 ml./min., and test No. 4 with a pickle liquor feed rate of 50 ml./min. There was a distinct odor of ammonia over the reaction mixture at all times inboth tests. The data were as follows:

Acid value, pickle liquor, 200.4 g. SOi/l. Iron value, pickle liquor, 60.0 g. Fe/l. Ammonia solution. 155.9 g. NH3/1.

.Volume of water in reactor at start of test, 8.0 1.

Volume of ammonia solution in reactor at start of test, 0.01.

These data show that when the ammonia was reduced to the level specified by Wilson, but not used by him in his example, an iron-free supernatant could be produced only where the pickle liquor feed was reduced to 50 ml./min., or 40% of the pickle liquor rate it should have been possible to use according to the Wilson patent.

Table I shows that, using my process in cyclic continuous operation, pickle liquor was fed at a rate of 320 ml./min. for 25 minutes. The feed period was followed by an oxidation period of 6 minutes and a drain period of 3 minutes, giving a total cycle time of 34 minutes. The net pickle liquor rate was, then, 236 ml./min. (320 25/34=236). Thus, with an excess of ammonia of slightly more than 2%, my process will convert pickle liquor at a rate of 236 ml./min. and produce an iron-free supernatant, whereas in the same size reactor Wilsons process converted only 50 ml./min. Thus the rate of pickle liquor conversion by my process is about five times that of Wilson (236/50=4.72).

I have duplicated the process of Wilson's Example II, which process is carried out without aeration, and have found that the settling rate of the precipitate was much slower than in my process. According t the Wilson example, only 38% by volume had settled after'20 minutes and it required 40 minutes to settle 60%, whereas according to my process, as shown in Table I, 92% settled in minutes.

Although this invention has been described as applied to the treatment of waste sulphate pickle liquor, it will be understood that it applies equally as well to the treatment of waste chloride pickle liquor, in which case the products of the reaction will be ammonium chloride and a rapidly settling magnetic iron oxide.

The above description of the apparatus and of typical runs are given as examples only, and are not to be construed as limiting the scope of the invention since various modifications may be made within the scope of the following claims without departing from my invention.

I claim:

1. A process for producing pure ammonium salt and a rapidly settling magnetic iron oxide, which comprises carrying out a first step b mixing streams of ammonia, an aqueous solution of ferrous salt and air in a reaction zone to form a bath, regulating said streams so that at the end of said first step the bath contains in solution from 0.1 gram to 5 grams of iron per liter and the bath contains precipitated iron oxide in which the atomic ratio of ferric iron to ferrous iron is less than 2, thereafter in a second step subjecting the bath to an oxidation treatment during an oxidizing period by agitating the bath and supplying air to it in a greater proportion relative to the ferrous salt than was used during the first step and until the atomic ratio of ferric iron to ferrous iron in the precipitated oxide is between 2:1 and 3.5:1 and until the bath contains substantially no iron in solution, said steps being carried out at atmospheric pressure and with an excess of ammonia above the stoichiometric requirements of said ferrous salt, and separating the precipitate from the solution.

2. A process according to claim 1, wherein the duration of'said second step is less than about 25% of said first step.

3. A process according to claim 1, wherein the duration of said second step is from 5 to 30% of said first step.

4. A process according to claim 1, wherein said second step is carried out without supplying ferrous salt solution to the bath.

5. A process for producing pure ammonium salt and a rapidly settling magnetic iron oxide,

. which comprises providing in a reaction vessel a slurry of iron oxide and ammonium salt solution, carrying out a first step by feeding streams of ammonia, an aqueous solution of ferrous salt and air to said slurr to form a bath, regulating said streams so that at the end of said first step the bath contains in solution from 0.1 grain to 5 grams of iron per liter and the bath con tains precipitated iron oxide in which the atomic ratio of ferric iron to ferrous iron is less than 2, thereafter in a second step subjecting the bath to an oxidation treatment during an oxidizing period by agitating the bath and supplying air to it in a greater proportion relative to the ferrous salt than was used during the first step and until the atomic ratio of ferric iron to ferrous iron in the precipitated oxide is between 2:1 and :1 and until the bath contains substantially no iron in solution, said steps being carried out at atmospheric pressure and with an excess of ammonia above the stoichiometric requirements of said ferrous salt, removing a portion of the slurry from the reaction vessel, treating the removed portion of slurry to separate the precipitate from the solution of ammonium salt, and utilizing the portion of the slurry remaining in the reaction vessel for repeating said first and second steps.

RICHARD D. HOAK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 961,764 Falding June 21, 1910 997,237 Carrick July 4, 1911 1,994,702 Harris Mar. 19, 1935 2,165,889 Fischer et a1 July 11, 1939 2,178,239 McKenna Oct. 31, 1939 2,419,240 Wilson Apr. 22, 1947 2,427,555 Elzi Sept. 16, 1947 FOREIGN PATENTS Number Country Date 1,050 Great Britain Nov. 8, 1906 433,333 Great Britain Aug. 13, 1935 Certificate of Correction I Patent No. 2,529,87f1 November 14, 1950 RICHARD D. HOAK It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

read

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 20th day of March, A. D. 1951.

THOMAS F. M'URPHY,

Assistant Gommiaioner of Patenta. 

1. A PROCESS FOR PRODUCING PURE AMMONIUM SALT AND A RAPIDLY SETTLING MAGNETIC IRON OXIDE, WHICH COMPRISES CARRYING OUT A FIRST STEP BY MIXING STREAMS OF AMMONIA, AN AQUEOUS SOLUTION OF FERROUS SALT AND AIR IN A REACTION ZONE TO FORM A BATH, REGULATING SAID STREAMS SO THAT AT THE END OF SAID FIRST STEP THE BATH CONTAINS IN SOLUTION FROM 0.1 GRAM TO 5 GRAMS OF IRON PER LITER AND THE BATH CONTAINS PRECIPITATED IRON OXIDE IN WHICH THE ATOMIC RATIO OF FERRIC IRON TO FERROUS IRON IS LESS THAN 2, THEREAFTER IN A SECOND STEP SUBJECTING THE BATH TO AN OXIDATION TREATMENT DURING AN OXIDIZING PERIOD BY AGITATING THE BATH AND SUPPLYING AIR TO IT IN A GREATER PROPORTION RELATIVE TO THE FERROUS SALT THAN WAS USED DURING THE FIRST STEP AND UNTIL THE ATOMIC RATIO OF FERRIC IRON TO FERROUS IRON IN THE PRECIPITATED OXIDE IS BETWEEN 2:1 AND 3.5:1 AND UNTIL THE BATH CONTAINS SUBSTANTIALLY NO IRON IN SOLUTION, SAID STEPS BEING CARRIED OUT AT ATMOSPHERIC PRESSURE AND WITH AN EXCESS OF AMMONIA ABOVE THE STOICHIOMETRIC REQUIREMENTS OF SAID FERROUS SALT, AND SEPARATING THE PRECIPITATE FROM THE SOLUTION. 